Abstract: Surface properties of K6509 Co-based alloy specimens cryogenic treated by -196 ℃ liquid nitrogen for 24 h and 48 h were studied and compared with non-cryogenic specimens by means of microstructure analysis, XRD component analysis and hardness analysis, and the changes of surface roughness and morphologies after cryogenic treatment and rolling polishing were analyzed. The results show that some of the carbides in Co-base alloy are roughened and the fine carbides are precipitated after 24 h cryogenic treatment, but after 48 h cryogenic treatment, the fine carbide decreases, the carbide coarsening disappears, and the microstructure becomes uniform. The XRD patterns show that the phase composition does not change during the cryogenic process, but the peak value shift at the eutectic phase containing M₂₇C₆ carbide and the eutectic phase containing M₇C₃ carbides, and the diffraction peak intensity of the M₂₇C₆ carbides changes significantly. The hardness is significantly increased by prolonging the cryogenic time, and the hardness is increased by 17.6% after 48 h cryogenic treatment. The combination of reasonable cryogenic time and rolling polishing can significantly improve the surface quality. After 48 h cryogenic treatment and rolling polishing, the surface roughness decreases the most, and the R_a value decreases from 1.215 μm to 0.662 μm, and the surface defects are significantly reduced and the surface consistency is well.

Key words: Co-based alloy, cryogenic treatment, surface morphology, microstructure, carbide, XRD